The present invention relates to an aqueous process for recycling acetal polymer, and moldings thereof, into formaldehyde monomer solution. The formaldehyde monomer solution may also contain formaldehyde trimer (hereinafter referred to as "trioxane").
Polyacetal resin is manufactured, in general, from formaldehyde monomer and/or trioxane. Formaldehyde-monomer-based polyacetal manufacturing processes are generally used to manufacture acetal homopolymers, while trioxane-based polyacetal manufacturing processes are generally used to manufacture acetal copolymers. Polyacetal resin is used in many applications and is molded into many types of products, such as disposable lighter bodies, ski bindings, conveyor chain links, gears, hinges, automotive window cranks, etc. After the useful life of such products expire, the product is scrapped, and becomes waste.
It is desirable, for economic and environmental reasons, to recycle polyacetal, and moldings thereof, back into the components from which it is manufactured, i.e., formaldehyde monomer and/or trioxane. One known way to recycle polyacetal is a gas phase recovery process, wherein the polyacetal is thermally decomposed with a catalyst to form formaldehyde monomer gas and then the gas is recycled directly into a polyacetal manufacturing process. Unfortunately, it has been found that the formaldehyde monomer gas generated by such a process may contain traces of volatile base products, such as, for example, amine compounds or possibly ammonia. When the formaldehyde monomer gas is recycled into a polyacetal manufacturing process, it can make the process inoperable by initiating polymerization in the wrong places. Furthermore, the yield obtained by the gas phase process is sometimes lower than desired. Thus, there exists a need to develop new and efficient means for recycling polyacetal resin into its monomeric formaldehyde and/or trioxane components, wherein said components are acceptable for use in an actual polyacetal manufacturing process.
In the present invention, there has been developed an aqueous process, done in the presence of an acid decomposition catalyst, for recycling polyacetal, and moldings thereof, into formaldehyde monomer solution. The formaldehyde monomer solution can, by the nature of the decomposition reaction, also contain trioxane. The resultant formaldehyde monomer solution is then treated (by, for example, filtration, neutralization, and purification processes) in preparation for cycling into formaldehyde-monomer-based polyacetal manufacturing processes. Alternatively, with or without further treatment, the resultant formaldehyde monomer solution can be reacted, either during the decomposition process or thereafter, to form trioxane, which in turn can be used in trioxane-based polyacetal manufacturing processes.